Conventional air conditioning devices work mostly by injecting cool air into an enclosed space in which cooling is desired. The air is injected in a way that results in mixing of the air in the space to achieve a relatively uniform temperature and perceived comfort level at any location in the enclosed space. Usually the air is injected by a fan in the air conditioner through one or more vents at relatively high velocity to create mixing throughout the enclosed space. In a displacement air conditioning system, the air is injected at the bottom of the space to create a cool air layer only in the lower section of the space occupied by people.
The air conditioner removes heat from the air by passing it through a “cold side” heat exchanger containing a cool fluid, or a heat exchanger cooled by some other mechanism such as the Peltier (or thermoelectric) effect. In this specification, the terms “evaporator” and “condenser” respectively refer to the cold side and the hot side heat exchangers. However the scope of the specification is not limited to compressor-refrigeration cooling.
The air inside the cooled space absorbs heat from the walls, floor, people and other objects inside the space being cooled.
Usually, but not always, the air inside the cooled space is recirculated through the cold side of the air conditioner to reduce the energy required to maintain cooling.
The heat absorbed from the cooled space air (including the latent heat obtained by condensing water vapour to liquid water) at the evaporator reappears at the hot side of the air conditioner. Outside air is passed through the condenser and increases in temperature as it absorbs heat from the condenser. The energy used to compress the refrigerant gas also appears at the condenser. Therefore the heat transferred to the warm outside air at the condenser is greater than the heat absorbed from the cooled space air at the evaporator by an amount equal to the electrical energy supplied to the compressor and fans (apart from relatively small amounts of heat lost from the system by other means). The coefficient of performance of the air conditioner is the rate at which heat is absorbed from the cooled space (including the latent heat obtained by condensing water vapour to liquid water) divided by the electrical power supplied to the compressor.
In essence the air conditioner operates as a heat pump, removing heat from air inside the cooled space in the cold side of the air conditioner and transferring this heat, along with the energy used to compress the refrigerant gas, to warmer air outside the cooled space in the hot side of the air conditioner. In the case of a split system air conditioner, the cold side and the hot side are physically distinct components at some distance from each other. In addition to the power required to run the compressor, a small additional amount of power is needed to run the fans to move the inside and outside air.
A portable air conditioner can be constructed from an air conditioner similar to known domestic air conditioners. The air conditioner is usually placed inside the room to be cooled and, therefore, a relatively large diameter air tube is required to ensure that hot air from the condenser is exhausted through a window. In some cases, a second air tube carries air from the window to the condenser circulation fan to be pumped through the condenser. The cool air mixes with the room air or, in the case of some inventions discussed below, is directed into a localized part of the room.
A substantial part of the energy used in these conventional air conditioning arrangements results only in cooling of the building structure and the objects inside the cooled space, and removal of heat entering through the roof or ceiling, walls, floor and particularly through open or covered apertures such as the windows and doors. This energy requirement can be reduced by providing additional insulation or by shading the roof, walls, windows and doors. However, these measures are not always possible, particularly with older buildings not designed with energy efficiency in mind.
By localizing the effect of an air conditioner to just a small section of the cooled space, typically away from doors, windows and walls, very large energy savings are possible. People often spend long periods of time at a single location within a room (such as sleeping on a bed) and it is only necessary to keep the upper body and face cooled for a person to feel very comfortable.
This principle has been described in U.S. Pat. No. 6,425,255 by Karl Hoffman, Dec. 26 2000 (issued Jul. 30 2002). Further refinements are described in US Patent 2002/0121101 by AsirlyaduraiJebaraj, 2 Jan. 2002 (issued 5 Sep. 2002). This patent also refers to China Patents CN2259099 (San Jianhua et al) and CN1163735 (Tan Mingsen et al) that describe air-conditioned mosquito nets in which outside air is conditioned and supplied to the enclosures and all of the air is exhausted outside the enclosure. China patent CN1061140 (He BaoAn et al) describes an insulating mosquito net with a plurality of inflatable air-pocket walls. Chinese developments also include localised air conditioning for seats in an auditorium.
These were preceded by U.S. Pat. No. 2,159,741 by C. F. Kettering et al, 30 Aug. 1933 (issued 23 May 1939) describes a fabric wall structure around the bed and a small air conditioning unit feeding air into the enclosed walled space over the bed. This invention exploited the displacement air conditioning principle in which it is known that cool air is denser than warmer air and thus remains in the walled enclosure over the bed.
Attempting to localize air conditioning by using a mosquito net, even with relatively fine weave, is inefficient. This difficulty was recognized in CN2803143Y in which the interior of the mosquito net is subdivided with an interior curtain such that only the head of the sleeping person is inside the air conditioned section. The slight density difference between cooler air inside the enclosure and the warmer air outside is sufficient to provide a pressure difference that will allow cool air to rapidly disperse through the net into the room. That is why many patents have disclosed impervious barriers to air flow. However, these can be unattractive for people who need to use the enclosure.
It is evident from the above that there is a need for a localised personal air conditioning system in which the conditioned air is used more effectively to cool a person located in a sleeping space.
Uninterruptible power supplies (UPSs) using battery storage have become popular in regions affected by frequent electricity supply interruptions because they are silent and emit no exhaust fumes. A typical UPS can supply power for several hours to operate low power fluorescent lights, communications equipment and a fan. Typical domestic UPS units can supply between 1000 and 2,500 Watts. In many markets, a high power UPS unit costs up to three times the price of the smallest air conditioner and often the batteries need to be replaced every twelve months or so.
An attractive alternative option is to supply power from a photovoltaic solar cell array through an inverter similar to those used for UPS units.
However, a typical UPS inverter cannot easily provide power for air conditioning. The reason is that the electric motor required to run the compressor (as used in a refrigeration air conditioner) draws up to ten times the normal electric supply current for a brief time, typically 50 to 100 milliseconds, when it starts operating from a stationary condition. While UPS units can supply a larger current for a short time without overloading, the power rating of the UPS unit needs to be about three times larger than the electric motor rating in order for the motor to start reliably. Therefore, one would need a UPS unit with a capacity in excess of 2,000 Watts to run even the smallest air conditioners rated at 600 Watts. Here it should be noted that some of the air conditioners said by their manufacturers to run at a relatively low power rating, for instance 450 Watts, actually require up to twice or two and a half times as much power under certain conditions, including when initially starting up. Therefore they typically cannot be run by a UPS system and instead require a generator that can supply the required power.
Many more people would be able to gain comfort and better sleep by using air conditioning if one could reduce the electric power required for the air conditioning compressor. This can be achieved by significantly reducing the cooling capacity required from the air conditioner. One way to do this is to localize the effect of the air conditioner so that only the air around the head and upper body is cooled.
A further, related problem also exists in the field. In order to achieve such a precisely localised cooling effect on a person from a reasonable distance, the cooling effect of a jet of air should be able to extend some distance from the origin of the jet. This is difficult because any turbulence in the jet is likely to promote mixing with the surrounding air, thereby reducing the velocity, and subsequently reducing the cooling sensation at the location of the person. As it turns out, the jet velocity at the location of the person is significant. For example, if the jet velocity exceeds 0.4 m/s, an additional apparent cooling of approximately 2° C. can be attained, due to the way in which the human physiology senses the apparent temperature of the surrounding air.
For a heat exchanger to operate at maximum heat transfer efficiency, a relatively uniform air velocity is required. If there is a large difference in air velocity in different parts of the heat exchanger, this reduces the effective heat exchange area, resulting in a greater temperature difference between the air in the evaporator tubes and the average temperature of the air after it passes through the heat exchanger. This means that more work needs to be done by a refrigeration compressor to achieve the same cooling effect.
The disadvantage of arrangements provided in the prior art is that air passing through the cooling side of the air-conditioner must be pushed through the evaporator heat exchanger by an air circulating fan. If a motor driven used to force air through the cold air side of an air-conditioner is located adjacent to the heat exchanger, it is difficult to achieve uniform air velocity through all parts of the heat exchanger because air leaves different parts of the fan at different velocities and sometimes different directions, depending on the design of the fan. Moreover, the air exiting the fan has significant vorticity, which can cause additional turbulence, causing the air jet to mix rapidly with the surrounding air.
In order to achieve a more uniform velocity, air-conditioner arrangements having the air pass through the heat exchanger before passing through the fan are often preferred. Undesired vorticity can be reduced through the provision of airflow straighteners. However, air flow straighteners known in the field present manufacturing challenges and have costly parts, taking up a relatively large amount of space. Any attempt to provide a practical, personal localised air conditioner is preferably compact and low-cost.
It is generally desirable to overcome or ameliorate one or more of the above mentioned difficulties, or at least provide a useful alternative.